1. Field of the Invention
This invention relates to oil and gas drilling and production equipment in general, and in particular, to an improved form of helical strakes which are useful for stabilizing floating, deep water offshore oil and gas drilling and production platforms.
2. Description of Related Art
Offshore oil and gas drilling and production operations involve the provision of a vessel, or platform, sometimes called a “rig,” on which the drilling, production and storage equipment, together with the living quarters of the personnel manning the platform, if any, are mounted. In general, offshore platforms fall into one of two classes, viz., “fixed” and “floating” platforms.
Fixed platforms comprise an equipment deck supported by legs that are seated directly or indirectly on the sea floor. While relatively stable, they are typically limited to relatively shallow waters, i.e., depths of about 500 feet (approximately 152 m), although one so-called “compliant piled tower” (“CPT”) platform, called the “Baldpate” tower, is said to be operating at a depth of 1648 ft. (approximately 500 m).
Floating platforms are typically employed in water depths of about 500 ft. (approximately 152 m) and greater, and are held in position over the well site by mooring lines anchored to the sea floor, or by motorized thrusters located on the sides of the platform, or both. Although floating platforms are more complex to operate because of their movement in response to wind and water conditions, they are capable of operating in substantially greater water depths than are fixed platforms, and are also more mobile, and hence, easier to move to other well sites. There are several different types of known floating platforms, including so-called “drill ships,” tension-leg platforms (“TLPs”), “semi-submersibles,” and “spar” platforms.
Spar platforms comprise long, slender, buoyant hulls that give them the appearance of a column, or spar, when floating in an upright, operating position, in which an upper portion extends above the waterline and a lower portion is submerged below it. Because of their relatively slender, elongated shape, they have relatively deeper drafts, and hence, substantially better “heave” characteristics, e.g., much longer natural periods in heave, than other types of platforms, and accordingly, have been a relatively successful platform design over the years. Examples of spar-type floating platforms used for oil and gas exploration, drilling, production, storage, and gas flaring operations may be found in the patent literature in, e.g., U.S. Pat. No. 6,213,045—Gaber; U.S. Pat. No. 5,443,330—Copple; U.S. Pat. Nos. 5,197,826; 4,740,109—Horton; U.S. Pat. No. 4,702,321—Horton; U.S. Pat. No. 4,630,968—Berthet et al.; U.S. Pat. No. 4,234,270—Gjerde et al.; U.S. Pat. No. 3,510,892-Monnereau et al.; and U.S. Pat. No. 3,360,810—Busking.
Despite their relative success, spar-type platforms include some aspects that need improvement. For example, because of their elongated, slender shape, they can be relatively more complex to manage during offshore operations under some conditions than other types of platforms in terms of control over their trim and stability. In particular, because of their elongated, slender shape, spar platforms are particularly susceptible to vortex-induced vibrations (“VIV”) resulting from strong currents acting on the hull of the platform. It is known that the provision of apparatus on the elongated hulls for vortex breaking, or controlled vortex-shedding, can reduce or eliminate this problem. For example, in U.S. Pat. No. 6,148,751 and U.S. Pat. No. 6,349,664, Brown et al. describe a hydrodynamic system for reducing vibration and drag on an elongated hull. In U.S. Pat. No. 6,244,785, Richter et al. describe elongated helical “strakes” disposed on a pre-cast concrete spar hull. Such prior art helical strakes typically comprise very heavy, helically formed, edge-supported plates that must be attached, e.g., by welding, to the hull while it is being fabricated in a dry dock.
While such prior art efforts at reducing or eliminating VIV of spar hulls do address the problem to some extent, they are also either relatively complex, and hence, expensive to implement, or heavy and difficult to assemble and incorporate into the hull efficiently. A need therefore exists for a simple, inexpensive, light-weight, easily assembled, yet effective apparatus for reducing or eliminating vortex-induced vibrations acting on the hulls of deep-water offshore platforms.